Typically, in playing back a signal recorded on an optical disk having a land/groove format using an optical head, the playback laser beam from the optical head experiences an optical phase difference between playback in a land and playback in a groove. Thus, the optical head requires optical compensation equipment for removing the optical phase difference in order to increase the quality of both playback signals for the land and the groove.
If the optical disk is inclined, or tilted (skewed), with respect to the light beam emitted from the optical head onto the optical disk, a spatial phase shift occurs in the playback laser beam spot, thus causing degradation in quality of the playback signals while increasing the occurrence of read errors. Thus, a compensator for compensating for such a phase shift, or an optical phase difference, must be provided in the optical head.
One compensator for compensating for such a phase difference that has attracted attention is a liquid crystal device.
A method for driving such a liquid crystal device in the related art is as follows:
A reference voltage (for example, a ground potential) is applied to one of two transparent electrodes that sandwich a liquid crystal therebetween, and a square wave with a duty ratio of 50%, of which the center of the amplitude is equal to the reference voltage, is applied to the other transparent electrode.
When the amplitude of the square wave is varied, the refractive index of the liquid crystal changes by ΔNLC. In this state, when laser light passes through the liquid crystal device, an optical phase difference of ΔNLC·d occurs, which is a product of the refractive-index change ΔNLC and the thickness d of the liquid crystal.
Generally, liquid crystals are damaged when a DC voltage (direct-current voltage) is applied. Thus, as described above, if one of the two transparent electrodes is fixed to a reference potential, a square-wave voltage (pulse voltage) with a 50% duty ratio, of which the amplitude center potential is equal to the reference potential, must be applied to the other transparent electrode.
In such a driving method of the related art, when only a single-polarity power supply (e.g., a positive power supply) is used, a reference potential circuit for generating the center potential of the square-wave amplitude is required.
If the reference potential is ground, two power supplies, that is, a positive power supply and a negative power supply, are required. A driving signal for driving the liquid crystal is a square wave, which is typically generated by a digital circuit, and the digital signal generated by the digital circuit is often single-polarity. A level-shifting circuit is thus required for generating a square wave whose reference potential is ground.
Therefore, there is a problem with the related art in that a complex method and apparatus for driving a liquid crystal device must be used to adjust the refractive index of the liquid crystal device, thus increasing the cost.
The present invention has been made in view of such a background, and it is an object of the present invention to provide a liquid crystal device, and a method and an apparatus for driving the same in which the refractive index of the liquid crystal device is adjusted using a simple driving method and apparatus, which is advantageous in that the cost is reduced.